In conventional chromatographic processes, a pulse of a feed mixture and a carrier fluid is passed through a column which is packed with an adsorbent. The adsorbent can be a porous or finely divided solid, or a granular material whose surface or internal pores provide adsorption sites or on which there has been deposited a film or coating of a desired non-volatile liquid adsorbent. The components of the feed mixture advance in the column with the carrier fluid, each at a rate which is related to the equilibrium process whereby the component partitions itself between the stationary adsorbent and the mobile carrier fluid. Each component under a given set of fluid and solid phase conditions has an effective partition coefficient defined by its respective concentrations on the stationary and in the mobile phases.
If the chromatographic column is viewed as being composed of discrete but contiguous narrow horizontal layers called plates, it is assumed that at each plate, equilibration of the component between the stationary and mobile phases will have occurred. According to such theory, the height equivalent of a theoretical plate, HETP, and the number of theoretical plates, N, are related by the equation: EQU N=L/HETP (1)
where L is the length of the solid adsorbent.
If the column is of sufficient length for complete separation of the components to occur, these pass from the column as separate fractions, the components of the feed that are held least strongly by the adsorbent exiting first.
Therefore in conventional chromatographic processes it is necessary that the adsorbent and the carrier fluid be carefully selected to provide a partition coefficient of the desired component which will enable separation from the other components of the mixture, and that the column be long enough for complete separation to occur.
Chromatographic processes are typically conducted as batch operations employing fixed adsorbent beds. However, the efficiency of batch operations is limited by column height and by the necessity of regenerating the adsorbent for use in subsequent separations. The continuous fixed bed processes which have been proposed are mechanically complicated, requiring the use, for example, of a rotating annulus or alternating injection of fluids.
Further, the efficiency of fixed bed processes is limited by the particle size of the adsorbent material. On the one hand, large adsorbent particles may resist molecular passage and can result in reduced chromatographic resolution. On the other hand, smaller particles, while providing better resolution, may result in a large pressure drop across the bed and thus require a reduction in fluid velocity.
While pressure drops associated with the use of smaller particles can be reduced by fluidizing a bed of such particles, separation efficiency is thereby compromised by the resulting backmixing and channel formation which are characteristic of such fluidized beds.
The use of a magnetically stabilized fluidized bed has been proposed for conducting various chemical processes, including separations. It will be understood that the term "magnetically stabilized fluidized bed" (abbreviated as "MSB") as used herein refers to a system wherein a bed of solid particles having a magnetizable component which are fluidized by the flow of a fluidizing fluid is stabilized against gross solids backmixing and fluid bypassing by the application of a magnetic field, such as is disclosed in U.S. Pat. No. 4,115,927, reissued as U.S. Pat. No. Re. 31,439, to Rosensweig, the entire disclosure of which is hereby incorporated by reference.
Rosensweig noted the quiescent, fluid-like behavior of a fluidized bed containing magnetizable particles which is subjected to an applied magnetic field substantially colinear with gravity, and further observed that such fluid-like behavior could be maintained over a wide range of operating velocities. These "superficial fluidization velocities" range between (a) a lower limit given by the normal minimum fluidization superficial fluid velocity required to fluidize or levitate the bed of solids in the absence of the magnetic field, and (b) an upper limit given by the superficial fluid velocity required to cause timevarying fluctuations of pressure difference through the stabilized fluidized bed portion during continuous fluidization in the presence of an applied magnetic field. As the superficial fluid velocity is increased, the pressure drop through the bed increases to a value corresponding to the ratio of bed weight to cross-sectional area at the minimum fluidization velocity, and then remains relatively constant as the fluid velocity is increased. The application of the magnetic field is said to make possible superficial fluidization velocities of 10 or more times the flow rate of the fluidized bed at incipient fluidization in the absence of the magnetic field, along with the substantial absence of bubbling in gas fluidized beds or roll-cell behavior in liquid-fluidized beds. This stably fluidized bed condition persists even as the solids are continuously added to and removed from the contacting vessel.
The following references report the use of the magnetically stabilized bed for conducting separations processes:
Coulaloglou et al., U.S. Pat. No. 4,247,987, which is hereby incorporated by reference, disclose a process for continuous countercurrent contacting of solids with a fluid stream in a magnetically stabilized fluidized bed, the solids descending in substantially countercurrent, plug-flow manner against the contacting stream. Coulaloglou et al. employ the process for continuous solids flow molecular sieve separations to absorb one species from a contacting fluid, the saturated adsorbent particles being continuously removed and then regenerated in a desorber to remove the adsorbed species.
Savage, U.S. Pat. No. 4,283,204, which is hereby incorporated by reference, discloses a continuous countercurrent adsorption process for separating contaminant components from a feedstream in a magnetically stabilized fluidized bed.
Siegell, U.S. Pat. No. 4,443,231, which is hereby incorporated by reference, discloses a continuous chromatographic separation in a magnetically stabilized fluidized bed wherein the bed particles continuously move transverse to the flow of the carrier fluid, which serves to fluidize the bed, such that the components of the feed mixture are transported downstream varying distances from the injection point depending on the adsorption and desorption characteristics of the components. Product streams comprising the carrier fluid and a portion of the feed mixture containing at least one of the components are recovered from the surface of the bed, with the most strongly adsorbed component being transported farthest from the injection point. Siegell also discloses modifications of the process to permit improved resolution of the components of the mixture by the addition of temperature programming or application of an electric field in a direction transverse to the flow of both the carrier fluid and the bed solids.
Reiter et al., in European Patent application No. 0083202 published July 6, 1983, show a magnetically stabilized bed being used for hydrocarbon separations. Reiter et al. also observe that the MSB process permits the use in chromatographic separations of small adsorbent particles having reduced diffusional resistance without incurring high pressure drop or gross fluid bypassing. Such particles are said to facilitate a more rapid transfer of the sorbed species from the fluid than do larger adsorbent particles, thereby enabling a faster approach to equilibrium.
However, in the above-recited magnetically stabilized fluidized bed processes column height places a limitation on the quantity of adsorbent available for separations. Further, these processes generally require development of tailored adsorbent-desorbent systems suitable for adsorbing with high specificity the component of the mixture sought to be separated.
A magnetically stabilized fluidized bed process for purifying and separating components of mixtures without the limitation of column height or the requirement of adsorbent specificity would therefore be highly desirable.